1. Field of the Invention
This invention relates to a cathode for an electron tube such as a cathode-ray tube of a TV set and particularly to an improvement in electron emission characteristics of the cathode.
2. Description of the Prior Art
FIG. 1 is a schematic sectional view illustrating a structure of a cathode for use in a cathode-ray tube (CRT) or an image pickup tube for a TV system. In a conventional cathode, a layer 2 of an electron-emissive substance made of an alkaline earth metal oxide containing at least BaO and further containing SrO and/or CaO is formed on a cylindrical base 1 made of Ni as a major element containing a small amount of a reducing element such as Si or Mg. A heater 3 is provided inside the base 1 and the electron-emissive layer 2 is heated by the heater 3 to emit thermal electrons.
Such a conventional cathode is manufactured by a process as described below. First, a suspension of a carbonate of an alkaline earth metal (Ba, Sr, Ca, etc.) is sprayed on the base 1 and the applied suspension is heated by the heater 3 in a dynamic vacuum. As a result, the alkaline earth metal carbonate is converted to an oxide. Then, the alkaline earth metal oxide is partially reduced at a high temperature of 900.degree. to 1000.degree. C. so that it is activated to have a semiconductive property, whereby an electron-emissive layer 2 made of an alkaline earth metal oxide is formed on the base 1.
In the above described activation process, a reducing element such as Si or Mg contained in the base 1 diffuses to move toward the interface between the alkaline earth metal oxide layer and the base 1, and then reacts with the alkaline earth metal oxide. For example, if the alkaline earth metal oxide is barium oxide (BaO), the reaction is expressed by the following formula (1) or (2). EQU BaO+1/2Si=Ba+1/2SiO.sub.2 ( 1) EQU BaO+Mg=Ba+MgO (2)
Thus, the alkaline earth metal oxide layer 2 formed on the base 1 is partially reduced to become a semiconductor of an oxygen vacancy type. Consequently, an emission current of 0.5 to 0.8 A/cm.sup.2 is obtained under the normal condition at an operation temperature of 700.degree. to 800.degree. C. However, in the cathode thus formed, a current density higher than 0.5 to 0.8 A/cm.sup.2 can not be obtained for the following reasons. As a result of the partial reduction of the alkaline earth metal oxide, an intermediate layer of an oxide or a composite oxide such as SiO.sub.2, MgO or BaO.SiO.sub.2 is formed in the interface region between the base 1 and the alkaline earth metal layer 2 as is obvious from the formulas (1) and (2), so that the current is limited by a high resistance of the intermediate layer. In addition, it is believed that the intermediate layer serves to prevent the reducing element in the base 1 from diffusing into the electron-emissive layer 2 so that a sufficient amount of Ba may not be generated.
Incidentally, in a cathode disclosed in Japanese Patent Laying-Open Gazette No. 20941/1984, the thickness of the base 1 is made thin to obtain a rapid response rate in reaction in the cathode and for the purposes of preventing exhaustion of the reducing agent during the lifetime of the cathode and preventing lowering of the strength of the base 1, lanthanum is contained in a dispersed manner in the base 1 in the form of LaNi.sub.5 and La.sub.2 O.sub.3.
A cathode formed by pressing powder of mixture of W and Ba.sub.3 Sc.sub.4 O.sub.9 is disclosed by A. van Oostrom et al. in Applications of Surface Science 2 (1979), pp. 173-186.
German Patent Laying-Open Gazette No. 2626700 discloses an electron-emissive substance for high-pressure discharge lamp where an alkaline earth metal oxide such as BaO is mixed with an oxide of W or Mo and a rare earth metal oxide.
British Patent No. 1592502 discloses an electron-emissive substance for a discharge lamp in which BeO and Y.sub.2 O.sub.3 are added to Ba.sub.2-x Sr.sub.x CaWO.sub.6 (x=0-0.5).